Operation of lithium‐based batteries at low temperatures (<0 °C) is challenging due to transport limitations as well as sluggish Li+ kinetics at the electrode interface. The complicated relationships among desolvation, charge transfer, and transport through the solid electrolyte interphase (SEI) at low temperatures are not well understood, hindering electrolyte development. Here, an ether/hydrofluoroether and fluoroethylene carbonate (FEC)‐based ternary solvent electrolyte is developed to improve Li cycling at low temperatures (Coulombic efficiency of 93.3% at ‐40 °C), and the influence of the local solvation structure on interfacial Li+ kinetics and SEI chemistry is further revealed. The hydrofluoroether cosolvent allows for modulation of the solvation structure, thereby enabling facile Li+ desolvation while forming an inorganic‐rich SEI, which are both beneficial for lowering Li+ kinetic barriers at the interface. This cosolvent also increases the oxidative stability of the electrolyte to over 4.0 V versus Li/Li+, thereby enabling cycling of NMC‐based full cells at −40 °C. This study advances the understanding of the influence of Li+ solvation structure, SEI chemistry, and interfacial Li+ kinetics on Li electrochemistry at low temperatures, providing new design considerations for creating effective low‐temperature electrolyte systems.
Lithium-ion batteries are ubiquitous in most energy storage applications, but they fail completely below about -20 ºC, which hinders their use in low-temperature applications, like electric aviation and space exploration. Materials that alloy with lithium offer high specific capacities and have been researched thoroughly at room temperature, but their low temperature behavior is not well understood, despite indications that they may exhibit superior performance. In this work, we characterize the electrochemical and structural changes of antimony, tin, and silicon anodes at temperatures down to -40 ºC using a tailored ether electrolyte. Antimony anodes exhibit reversible cycling at -40 ºC, with a first cycle specific capacity of 450 mAh g-1, and tin and silicon can be cycled at -20 ºC with first cycle specific capacities far exceeding that of graphite. In addition, three-electrode coin-cell experiments reveal strong influences of the lithium metal counter electrode on the overall galvanostatic voltage profiles; these influences become dominant at low temperature and obscure the inherent behavior of the alloy anodes. This finding highlights the importance of using an appropriate reference electrode for low-temperature experiments, especially for sensitive experiments such as the galvanostatic intermittent titration technique (GITT). GITT experiments revealed the contributions of kinetic processes and thermodynamic hysteresis as a function of temperature in these materials, and they showed that the different materials feature different kinetic and hysteretic behavior at low temperature. The low temperature electrochemical behavior of silicon is seemingly more dependent on structural changes during the lithiation/delithiation transformations, while tin and antimony are more strongly impacted by kinetic effects at low temperatures. Finally, X-ray diffraction experiments on antimony anodes reveal difference in the average crystallite size and crystallite evolution at low temperature during cycling. Overall, these results are an important for understanding the fundamental behavior of alloy materials at low temperature and are an important step for the engineering of high-energy, low-temperature lithium-ion batteries.
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